Extracting subset of mp3s from large NAS

[windbag]

I have an expanding music database as I rip my CDs to FLAC & mp3 files to my NAS, as
Artist/Album/FLAC/xx.flac & Artist/Album/xx.mp3, with the Artist folders set in a particular structure.
This enables me to play either type via Sonos, but I’d also like to play mp3s in the car, which has a limited capacity via card (32GB).

I can extract just the mp3s using FreeFileSync, which does a great job – but there are already too many mp3 files for the card capacity.
So I need to flag a subset of folders as my “prime listening” to keep within the card capacity.
I’ve tried labelling some as “not4car” in the filename, which works, but is a very longwinded job, as the majority of albums (eg compilations, live etc) need so marking.
Even marking the prime ones by adding “prime” or some-such to the filenames is laborious and makes the filenames look naff.

I’d like to find a way of tagging “prime” folders so that I can extract them on the basis of that tag when I make a subset-copy for the car SD card.
I can’t see anything in FreeFileSync that could help.

Anyone know a quick way to achieve this?

[Plerry]

The only option available in FreeFileSync (FFS), is exclude non-"prime" files and/or folders to the FFS Exclude Filter.

[windbag]

Thanks for the reply.
Yes, as I said, I've done that by adding a designated phrase to the filenames, such as "not4Car" & putting that in the FFS filter designation, but it is time consuming and cumbersome.
The solution doesn't have to be achieved by FFS - happy to use any other free utility to do the job.
Have thought about mp3 tags, but unsure how easy to apply/use for selection. It would be much faster to apply to folders - which may exclude mp3 tags, which seem to be for files only.